A gear type transmission having a plurality of speed change clutches incorporated therein is so constructed that change of speed stages is carried out by making selective engagement among the speed change clutches and therefore so-called speed change shock occurs during engagement among the speed change clutches. As is well known for any expert in the art, the larger an amount of slippage of the speed change clutches, the smaller the speed change shock. On the other hand, increase of an amount of slippage leads to increase of an amount of heat generated by the speed change clutches. Due to the fact that construction machine such as dump truck or the like has considerably high intensity of torque transmitted by the speed change clutches compared with that of passenger car or the like, a large amount of heat is generated by slippage.
In view of the above-mentioned fact construction machine is so designed that clutch hydraulic pressure is caused to increase gradually (in the form of modulation) to such an extent that an amount of slippage does not increase excessively.
Torque transmitted by means of the speed change clutches varies considerably in dependence on the running state of a vehicle. Specifically, torque to be transmitted in the case where an article having heavy weight is loaded on the vehicle is remarkably different from that in the case where no article is loaded thereon.
Since clutch hydraulic pressure is hitherto caused to increase gradually with the use of mechanical type modulation valves, it results that clutch hydraulic pressure has fixed gradual increase characteristics and thereby this leads to an occurrence of such malfunctions that a high intensity of speed change shock appears under a certain running state of vehicle and an amount of slippage increases unnecessarily.
One of objects of the present invention is to inhibit an occurrence of malfunctions as mentioned above.
On the other hand, a conventional transmission in which clutch hydraulic pressure is caused to increase gradually with the use of mechanical type modulation valves has the following drawbacks.
Specifically, FIG. 27 schematically illustrates an example of a gear type transmission employed for dump truck or the like. As is apparent from the drawing, the transmission is equipped with speed change clutches 401, 402, 403 and 404 for selecting one of speed stages comprising first speed, second speed, third speed and rearward movement.
It has been found with respect to the conventional transmission as constructed in the above-described manner that so-called speed change shock occurs when hydraulic pressure is abruptly exerted on the speed change clutches 401 to 404. To obviate this problem the transmission is so constructed that a mechanical type modulation valve 410 and a quick return valve 411 are interposed between switching valves 405 to 408 hydraulically connected to the speed change clutches 401 to 404 and a hydraulic pump 409.
The modulation valve 410 and the quick return valve 411 have a function for gradually increasing hydraulic pressure, for instance, in the clutch 410 in the purely mechanical manner (in the form of modulation) when a switching valve 405 is shifted by activating an electromagnetically actuated valve 412 for the purpose of controlling of pilot pressure which is operatively connected to the switching valve 405. By virtue of the function as mentioned above hydraulic pressure exerted on the clutch 401 is caused to increase with a constant gradient (time change rate), resulting in speed change shock being reduced. When any one of the electromagnetically actuated valves 413, 414 and 415 is activated, clutch hydraulic pressure in the speed change clutch operatively associated with the thus selected electromagnetically actuated valve is caused to increase gradually.
Now, consideration is taken into account, for instance, on the case where speed change is carried out from first speed to second speed. In this case the electromagnetically actuated valve 413 is activated at the same time when the electromagnetically actuated valve 412 is deactivated. As a result, hydraulic pressure exerted on the speed change clutch 401 decreases from a predetermined pressure to a level of zero, as shown in FIG. 28(a). On the other hand, hydraulic pressure exerted on the clutch 402 is caused to increase gradually under the effect of the modulation valve 410 and the quick return valve 411 after filling time to be described later elapses, as shown in FIG. 28(b). At this moment output torque from the transmission varies as typically shown in FIG. 28(c).
The filling time by which clutch hydraulic pressure is maintained at the substantially zero state is generated when hydraulic oil flows rapidly into the clutch pack of the speed change clutch 402 which is kept in the empty state and this filling time is over at the time point when the clutch pack is fully filled with hydraulic oil.
It is natural that during the period of filling time torque transmitted from the transmission is reduced to a level of zero. For the reason it is preferable that this filling time is short as far as possible and it is ideal that it is not existent. Thus, such a technical idea that a large amount of hydraulic oil is fed into a speed change clutch to shorten filling time is thinkable. However, to realize the above-mentioned technical idea there is a problem that a hydraulic pressure mechanism is constructed in larger dimensions. It is impossible to completely eliminate filling time in spite of the fact that the hydraulic pressure mechanism is designed and constructed in larger dimensions.
Another object of the present invention is to obviate the problem which is caused by filling time by which transmitted torque is reduced to a level of zero without any necessity for designing the hydraulic pressure mechanism in larger dimensions.
Another object of the present invention is to inhibit an occurrence of speed change shock by equalizing output torque from the transmission just before speed change to output torque from the same immediately after speed change.